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11-14-2018, 04:24 AM   #1
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PTTL vs Manual Mode, Full Power Output

Just a quick question.

Ok, let's say we have a Cactus V6ii with a RF60x. You can use Manual control of the flash or PTTL (from the V6ii). With PTTL you can toggle up to +5 (on the V6ii unit), and in Manual Mode select '1' as max power (from the v6ii unit). When shooting are they both the SAME power output, i.e max, i.e.e. +5 PTTL = 1 Manual Mode? Or is PTTL doing it's preflash thing and although you have said 'go maximum' with it (ie +5) it's still 'holding back' because it wants to somewhat expose the scene correctly, whereas Manual Mode ignores all that and 1 is max power?

TIA!

Bruce

11-14-2018, 04:31 AM   #2
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I don't have a V6II but I do have a V6 and RF60, and I didn't know it is possible to set flash power manually with using automatic flash power setting with PTTL. I thought that when using PTTL, change of lighting is done via flash exposure compensation setting only, from the auto exposure baseline. But since I don't have the V6II , maybe I'm wrong.
11-14-2018, 05:05 AM   #3
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QuoteOriginally posted by BruceBanner Quote
When shooting are they both the SAME power output, i.e max, i.e.e. +5 PTTL = 1 Manual Mode?
PTTL + 5 is not maximum power output. It is the computed correct exposure plus 5 stops.

Logic tells me that if the PTTL calculated exposure is say two stops below the maximum the flash can produce, then the +5 compensation will only be able to output +2 stops. But those with a greater knowledge than I may be able to explain more.
11-14-2018, 06:41 AM   #4
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My understanding from my review (not fully internalized) of the Cactus user guides, along with various descriptions provided by, e.g., mcgregni, Class A, is that the P-TTL process will attempt, on the basis of the pre-flash, to produce a "correctly" exposed image. The V6II can be used to adjust the relative balance among the light energy (lumen seconds) outputs from the various flashes in use, but the total illumination on the subject will be the same. To overexpose or under expose requires changing the exposure setting on the camera. It is the camera that reads the net received light from the sum of the pre flashes and decides how long the flashes will be allowed to operate before the flash tubes are quenched. Within the guts of any flash there may be more sophisticated approaches to flash power control, but ultimately the camera is (it thinks) commanding the flash energy.

11-14-2018, 06:47 AM   #5
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Maximum flash power ratings are difficult to measure on ones own, different manufacturers have different metrics which usually stop well short of the flash bulb physically detonating. Adding in flash power compensation won't increase power if it is already maxed out: the ultimate limit is the flash capacitors, which can only dump so much energy into the bulb*.


*and also they have narrow limits on how high their operating temperatures can get before they lose efficiency or physically self destruct.
11-14-2018, 06:53 AM   #6
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Manual 1/1 power is the only way to be sure the flash will operate at maximum power. A +5 FEP only means it will use the power needed to produce a 5 stops overexposure. Which doesn’t necessarily equate to full power.
11-14-2018, 01:31 PM - 1 Like   #7
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This is one of those questions that is possible to answer in so many ways ....I think we're all trying to say the same thing!

Here's my attempt .....Manual power levels are always fixed light output amounts, specific to each flash (and exactly how much light 1/1, 1/4, 1/16 produces depends on the Guide Number).

Flash compensation settings are stop based "offsets" against the fluctuating base exposure value decided by the metering pre-flash. This base exposure is a moving thing, potentially different for every frame.

If the base flash exposure value needed is very low, say when working at close distances, wide aperture and high ISO, then there is a chance that a flash could produce a compensated output 5 stops over the base before it reaches it maximum power output.

In the real world, at typical distances, apertures and ISOs, it is unlikely that a flash could provide 5 stops overexposure before reaching its limits .... It'd say about 2 or 3 stops over is as much as we might expect for a typical GN 54 unit in typical scenarios (if used nicely and not just blasted directly at the subject).

Of course, in theory, why would we need or want 5 stops of plus flash compensation ...? That would just be horrible, a washout.
11-14-2018, 06:43 PM   #8
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Thanks guys, yes this makes sense now. I was just in the garden last night practising with 'Davo' (my mannequin), and I was shooting through a variety of modifiers etc, and I was seeing the difference between +5 in TTL mode vs 1/1 Manual, both were at times a horrible washout, but hard to tell at times which more. I'm looking at how much the diffusers impact the light hitting the target and that sorta thing, seeing how far back I could have the light stand from Davo with a modifier etc.

Cheers!

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